A. Field of the Invention
The present invention relates to the field of cancer therapy, specifically, the treatment of chronic myelogenous leukemia. More particularly, these treatments involve the use of antisense oligonucleotides and liposomal formulations thereof.
B. Related Art
Chronic myelogenous leukemia (CML) is a hematologic malignancy in which uncontrolled proliferation of granulocytes occurs. It often is characterized by the reciprocal translocation of chromosomes 9 and 22, which relocates the Ableson (abl) protooncogene onto the 3′-end of the breakpoint cluster region (bcr). This produces a chimeric bcr-abl gene encoding a p210bcr-abl fusion protein, which is tumorigenic and is necessary for the growth of CML cells (Szczylik et al., 1991; Skorski et al., 1994; Tari et al., 1994; McGahon et al., 1994; Bedi et al., 1994).
The bcr-abl protein can autophosphorylate at the 177 tyrosine amino acid found within the first exon of bcr. When phosphorylated, the bcr domain of the bcr-abl protein binds to the SH2 domain of the growth factor receptor-bound protein 2 (Grb2) adaptor protein. Through the SH3 domain, Grb2 binds to the human Son of sevenless 1 (hSos1) GDP/GTP exchange factor resulting in ras protein activation. The bcr-abl protein can also transphorylate the 177 tyrosine amino acid found within the normal bcr protein. It is believed that when the normal bcr protein becomes tyrosine phosphorylated at amino acid 177, it also will complex with Grb2. When the bcr-abl protein is expressed, the p46 and p52 Shc (Puil et al., 1994) proteins become tyrosine phosphorylated as well. These Shc proteins have also been shown to form stable complexes with Grb2. Therefore, Grb2 appears to play a very important role in the tumorigenicity mediated by the bcr-abl protein (Puil et al., 1994; Pendergast et al., 1993).
Another adaptor protein, Crk-like (Crkl), also has been found to bind to bcr-abl. Unlike Grb2, Crkl binds to bcr-abl through the abl domain. Through its SH3 domain, Crkl can also bind to hSos1, which again leads to Ras protein activation (ten Hoeve et al., 1994a and 1994b). Thus, via the Grb2 and Crkl adaptor proteins, the bcr-abl protein has been linked to ras activation, which is known to lead to tumorigenesis. When ras protein expression is inhibited, proliferation of CML cells is also inhibited. Therefore, one of the major pathways in which bcr-abl protein promotes CML proliferation is by activating ras protein (Skorski et al., 1994 and 1995).
Liposomal-antisense oligonucleotides targeted to bcr-abl can reduce proliferation of CML cells. After maximal inhibition (75% growth inhibition and 90% protein inhibition), CML cells can still recover and grow if the liposomal-antisense oligonucleotides are removed from the culture medium. This is likely caused by incomplete inhibition of bcr-abl protein synthesis Tari et al., 1994). Thus, despite the ability of antisense oligonucleotides to inhibit proliferation of CML cells, there remains a need for more effective compositions and treatments against this form of cancer.